This invention relates to excellent composite materials for containers having excellent flavor-retaining property, excellent sealability and easy openability. More specifically, this invention relates to composite materials for packaging containers, for example composite materials used as packing materials for caps of glass bottles, polyethylene terephthalate (to be referred to as PET) bottles and metallic cans, composite materials for closures used in cups such as plastic cups and paper cups, composite materials for closures of straw-inserting holes of plastic standing pouches, composite materials for closures of composite containers, and composite materials for can bodies.
To secure accurate sealability, conventional caps for packaging containers are equipped with a packing material obtained by providing a cushioning material such as foamed polyethylene sheet on the inside surfaces of such caps as desired, and laminating thereto a single or a composite material such as a polyethylene film, a PET film, a film of a partially saponified ethylene/vinyl acetate copolymer, an aluminum foil, etc. (see, for example, Japanese Laid-Open Patent Publication Nos. 148687/1979 and 148688/1979). Composite materials composed of an aluminum foil and polyethylene, etc. laminated thereto have been used in heat-sealable closures of plastic cups or polyethylene-coated paper cups for packaging puddings, wines and liquors. Since, however, conventional materials do not have entirely satisfactory sealability as packing materials for caps of packaging containers, leakage of carbon dioxide gas occurs frequently. In the case of using such materials in hot packs, air comes into the containers which are under reduced pressure, and causes degeneration of the contents. When these materials are used in the cups, their excessively high heat seal strength makes it difficult to open the containers. It has therefore been desired to secure easy openability. But since improving the openability has resulted in reduced sealability, no entirely satisfactory means has been discovered for providing good sealability and easy openability simultaneously. Furthermore, plastic materials such as polypropylene as composite materials for composite can bodies have poor gas-barrier property, and attempts have been made to improve their gas-barrier properties by lamination of a material having gas-barrier property. But such a laminated material has the defect of being poor in closure sealability.